Method for applying zinc phosphate coatings to metal surfaces

ABSTRACT

Zinc and zinc/calcium phosphate coatings of excellent quality can be formed on ferriferous and zinciferous surfaces without an intermediate water rinse between alkaline cleaning and phosphating if cleaners containing polyphosphates or sequestrants are avoided and the soil content of the cleaner bath is maintained below about 5g/1.

United States Patent Schneider June 25, 1974 [5 METHOD FOR APPLYING ZINC 2,874,081 2/1959 Cavanagh et a1. 148/6152 PHOSPHATE COATINGS o METAL 3,144,360 8/1964 Palm 148/6.152 SURFACES 3,510,365 5/1970 Rausch et a1. 148/6.15 R

[75] Inventor: George Schneider, Trevose, Pa. P E Ed d G wh b n'mary xaminerwar it y [73] Asslgnee' Amchem Products Ambler Attorney, Agent, or Firm-Synnestvedt & Lechner [22] Filed: Dec. 28, 1971 [21] Appl. No.: 213,203

[57] ABSTRACT [52] US. Cl l48/6.l52, 1 17/127, 117/ 169 R, i d i l i m phosphate coatings of excellent 134/27 134/28 134/29 quality can be formed on ferriferous and zinciferous [51] Int. Cl. C23f 9/02 surfaces without an intermediate water rinse between [58] Field of Search 134/27, 28, 2 lk li cleaning a d phosphating if cleaners contain- 148/6.15 R, 6152, 6.16; 117/ 127, 169 R ing polyphosphates or sequestrants are avoided and the soil content of the cleaner bath is maintained [56] References Cited below about 5 UNITED STATES PATENTS 2/1943 Jernstedt 148/6.152

9 Claims, N0 Drawings METHOD FOR APPLYING ZINC PHOSPHATE COATINGS TO METAL SURFACES This invention relates to methods for applying zinc and zinc/calcium phosphate coatings to metal surfaces, particularly ferriferous and zinciferous surfaces, including in the latter case, galvanized surfaces.

Zinc phosphate coatings have long been used to improve paint adhesion and corrosion resistance properties of metal surfaces, especially steel and galvanized surfaces. In mass production facilities, such as an automobile body assembly line, such coatings are typically applied in a multi-step process using multi-stage equipment for contacting the metal objects with various treating liquids are they are moved along the assembly line. Although there are wide variations in the arrangements of equipment in various commercial installations and corresponding variations in the particular set of process steps practiced, a zinc phosphating line typically includes the following:

a. one or more alkaline cleaning stages (succeeding stages may use the same or different cleaners),

b. a grain refining stage (this stage is sometimes omitted or combined with the last cleaning stage),

c. one or more water rinse stages (for removal of soil and cleaner residue),

d. one or more zinc phosphating stages,

e. one or more water rinse stages (for removal of excess phosphating solution), and

f. one final acidulated rinse stage (for sealing the coating).

The sequence of treating is in the order given above. Each stage involves a substantial amount of equipment, whether it is one of the chemical treating stages (a, b, d, and f) or one of the rinsing stages (c and e). Two basic types of stages are employed: a spray stage includes a liquid reservoir, a spray tunnel above the reservoir, spray risers and pumps; a dip stage includes a treating tank large enough to hold at least one of the metal objects being treated, equipment for moving the objects into and out of the tank, and agitation equipment. In more modern installations, each stage has associated with it automatic or semi-automatic control and/or monitoring equipment. Rinse stages are most often of the spray type. Even though the process step being performed in a rinse stage is simpler than that being performed in a chemical treating stage, the amount of equipment involved is substantially the same.

The rinse stage (stage c) occurring after alkaline cleaning and prior to zinc phosphating has heretofore been regarded as of great importance in the satisfactory and economical operation of the overall process and has also been regarded as of great importance to the production of high quality zinc phosphate coatings.- As it has been traditionally practiced, stage c has involved the use of copious amounts of water. It has also involved the creation of copious amounts of dilute waste water which has been flushed to the sewer, either directly from the rinse stage, or indirectly through overflow into a cleaning stage (stage a). Neither circumstance was of great concern in the past, when the primary criterion of a good process was the economical production of zinc coating of the best possible quality. More recently it has been necessary to include environmental considerations in the evaluation of the satisfactoriness of industrial processes, and when the prior practice of rinsing after alkaline cleaning and prior to zinc phosphating is viewed with such considerations in mind, it is a much less desirable and satisfactory process step. Environmental considerations make it desirable to minimize the industrial use of water, and make it necessary to treat the water which is used to 'purify it before it is disposed of.

The treatment of dilute waste water from a rinse following alkaline cleaning (stage c) is a troublesome operation because of the large volume of water which must be handled, because of the difficulty in breaking the dilute emulsion of oil in the water for removal, and because of the difficulty in precipitating the dilute suspension of dirt and soil for removal.

Other disadvantages inhere in the use of rinse stages between alkaline cleaning and zinc phosphating. As pointed out above, each rinse stage, like every other stage, involves equipment requiring capital investment and floor space, as well as operational attention and expense. Another disadvantage involves the phenomenon known as rust blushing of steel parts. Rust blushing occurs in an assembly line during shutdowns, when the parts hang in hot humid spray cabinets for varying periods of time until the line is restarted. The steel parts, especially those which have been cleaned and rinsed but not yet coated, have highly activated surfaces, and a fine, light rust tends to form on them very rapidly. The rust blush interferes in varying degrees with later surface finishing, especially painting. Elimination of a rinse stage would reduce the overall length of the spray tunnel, and thus reduce the number of parts subjected to rust blushing during any given shutdown. Further, elimination of a rinse stage after alkaline rinsing would eliminate the presence of parts most likely to rust blush those which have been cleaned and rinsed, but not yet coated.

Despite the foregoing considerations, rinsing after alkaline cleaning continues to be regarded as important to satisfactory zinc phosphating, because cleaner and soil residue tend to degrade coating quality and uniformity.

It is an object of this invention to provide a method which produces zinc or zinc/calcium phosphate coatings on ferriferous and zinciferous surfaces without a water rinse after the alkaline cleaning step. 7

Related objects of the invention include provision of a zinc phosphating method which requires less equipment for operation, which is less plagued by rust blushing, which reduces water consumption, and which produces a waste stream which is more readily treatable to eliminate pollutants.

The invention is based on the discovery that phosphate coatings of excellent quality can be produced without rinsing after alkaline cleaning if alkaline cleaners containing significant amounts of polyphosphates or sequestrants are avoided and the alkaline cleaning bath is replaced when its soil content reaches a defined level or is otherwise maintained with a soil content below said defined level. Batch replacement of the cleaning bath produces a small volume, easily treated waste stream, instead of a continuing stream of dilute sewage.

The invention may be practiced with zinc and zinc- /calcium phosphating solutions of the type known to the art, and the use of two such solutions will be illustrated in the examples which follow. The operating conditions of the phosphating bath remain within conventional parameters of Zinc concentration, pl-l, free and total acid, temperatures, and treating time.

The alkaline cleaners which may be used in the practice of the invention include all those which contain substantially no polyphosphates or sequestrants such as ethylenediaminetetraacetic acid (EDTA) or its salts. Orthophosphate based cleaners, caustic based cleaners, and silicate based cleaners are illustrative of the types which may be employed. The cleaners are employed at concentrations and temperatures conventionally used in the art. As pointed out above, activating materials (Jernstedt salts complex titanium salts) are often used prior to zinc phosphating, and in accordance with the invention, they may be incorporated into the cleaner, as in the case of mildly alkaline cleaners, or fed separately and substantially continuously into strongly alkaline cleaners, such as caustic cleaners, or applied in a separate step between alkaline cleaning and zinc phosphating.

Following the application of the zone phosphate coating, the treated surfaces are water rinsed and/r given a final acidulated rinse.

The reason that polyphosphates and sequestrants are to be avoided in accordance with the invention is that they inhibit the formation of the zinc phosphate coating, leading to reduced coating weight, and consequent lack of uniformity of coating properties during the course of a run as their concentration builds up in the coating bath through drag-in, and ultimately bringing about a deterioration of the corrosion resistance and paint adhesion properties of the coating.

in accordance with the invention, the soil content of the alkaline cleaning bath is maintained below Sg/l of soil. Soil is defined for the purpose of this criterion as the amount of material extractable by benzene from a sample of the working cleaner bath after the sample has been neutralized with sulphuric acid. The maintenance of the cleaning bath according to this criterion may be accomplished in several ways. The preferred method is to monitor the bath by taking periodic soil tests, and dumping it and making up a fresh bath when the soil concentration reaches 5g/l. Alternately, if it has been determined that a working bath accumulates 5g/] of soil in a specified period of time, then part of the bath can be removed periodically and replenished with fresh cleaning solution. In this manner, the upper limit of Sg/l will never be reached, and the effluent from the system can be more systematically handled.

The practice of the invention may be further understood from the following detailed examples.

Example 1 In this test a zinc/calcium phosphate coating solution was employed and an ortho-phosphate cleaner was used. Only three stages were utilized: alkaline cleaning, phosphating, and water rinse. Steel panels were treated and then painted and scribed and thereafter subjected to 336 hours of salt spray exposure. Under paint corrosion resistance was evaluated by measuring the loss of paint at the scribe mark and numerical rating based on a scale of was assigned.

The cleaner employed in the first stage had the following composition:

Caustic soda 10 Soda ash 35 Continued Trisodium phosphate (anhydrous) 26 Filler (sodium sulfate) 29 Pounds Calcium carbonate (98.5%) 1.1 8.8 Zinc oxide 1.0 8.0 phosphoric acid 3.6 38.8 38 Be nitric acid 2.9 23.2 water 3.9 31.2 12.5 100.0

The foregoing concentrate was employed at a dilution of 2% by volume. The treating bath was accelerated with nitrite ion in conventional manner. The treating temperature was 150 F and the treating time was 60 seconds.

The panels were then rinsed at 65 F for 60 seconds. The salt spray exposure results were as follows:

Table 1 Loss at scribe Rating trace I0 Mild steel plating stock Automotive stock Galvanized panels were also treated in the foregoing three-stage process test, but as was expected, their performance was poor. Zinc/calcium phosphating baths of the type here employed areknown to produce inadequate results on galvanized surfaces.

Example 2 In this example a four-stage system in accordance with the invention was compared with a conventional six-stage system utilizing a water rinse prior to phosphating. Steel and galvanized panels were treated, painted, scribed, and exposed to 168 hours of salt spray.

The four-stage system included a cleaning step followed immediately by a phosphating step, a water rinse step, and an acidulated final rinse step. The six-stage system used for comparative purposes included a cleaner step, a water rinse step, a grain refining step, a phosphating step, a water rinse step and an acidulated final rinse step. The same cleaner, the same phosphating solutions and the same final rinse solutions were used in both the four-stage process and the six-stage process.

The cleaner employed was the same as that of Example l and the treating conditions were the same. In the four-stage process a grain refining material consisting of percent by weight disodium phosphate and 5 percent by weight potassium fluotitanate was continuously added to the cleaner to maintain a concentration therein of about 0.6 g/l (0.06 percent). In the six-stage process the same grain refining material was employed in the third (grain refining) stage at a concentration of it by weight Zinc oxide 12.5 75% phosphoric acid 58.] Nickelous oxide (75% Ni) 1.1 Ferric chloride hexahydrate .2 Sodium chlorate 3.9 Water balance The foregoing concentrate was employed at a dilution of 1V2 percent by volume, with a treating time of 60 seconds and a treating temperature of l25-l 30 F.

The dilute chromate final rinse was made from a concentrate having the following composition:

Pounds 70 Chromic Acid 5.007 42.1 Formaldehyde (37%) 0.886 7.5 Water 5.998 50.4

and was employed at a dilution of 0.1 percent by volume.

The salt spray results for steel and galvanized panels were as follows:

Table 11 Loss of Scribe Rating Four stage Steel 3/32 6.5

Galvanized H3 2 8 Six stage Steel 3/16 5 Galvanized 1/32 8 Example 3 An additional test illustrates a four-stage process in accordance with the invention utilizing two cleaners, the first being a strongly alkaline cleaner, high in caustic and orthophosphate, and the second being a mildly alkaline cleaner which also contains the activating or grain refining material (Jernstedt salt).

In the four-stage system of this example the first stage was application of the strong alkaline cleaner, the second stage was the application of the mild but activated alkaline cleaner, the third stage was application of' a I zinc phosphate coating, and the fourth stage was appli- 6 tion of 7.5 g/l, at a temperature of 160 F, and at a treating time of seconds. Its soil content was held below 5 g/l.

The mild activated cleaner employed in the second stage had the following composition:

K by weight Disodium phosphate 67 Activator (95% disodium phosphate,

5% fluotitanate) 17 Sodium bicarbonate 8 Surfactants 8 This cleaner was used at a dilution of 2.4 g/l, at a temperature of 150 F, at a treating time of 60 seconds, and with a soil content below 5 g/l.

The zinc phosphating solution employed in the third stage was the same as that employed in Example 2. It was employed at a dilution of 1 /2 percent by volume, with a treating time of 60 seconds and a treating temperature of F.

The acidulated rinse in the fourth stage was made from a concentrate having the following composition:

and was employed at a dilution of 0.05 percent by volume.

The salt spray results for steel and galvanized panels were as follows:

Table 111 Primer & Gilsonite Electroprimer Top Coat 336 hrs. 168 hrs. 240 hrs.

Steel 7 r 8.5 10 Galvanized 6 7 6.5

From the foregoing examples it can be seen that in accordance with the present invention excellent phosphate coatings can be obtained while at the same time water consumption and water pollution are materially reduced by the elimination of the conventional intermediate water rinse.

1 claim:

1. A method for applying a phosphate coating from the class consisting of zinc and zinc/calcium phosphate to ferriferous and zinciferous surfaces comprising: cleaning said surfaces by contacting them with an aqueous alkaline cleaner substantially free of polyphosphates and sequestrants, thereafter, without applying an intermediate water rinse, contacting said surfaces with a phosphating solution selected from the class consisting of zinc phosphating and zinc/calcium phosphating solutions to form a phosphate coating thereon, thereafter rinsing the coated surfaces with water, and maintaining said alkaline cleaner with a soil content below about 5 g/l, as measured by extraction in benzene following neutralization with sulphuric acid.

2. A method according to claim 1 wherein the water used to rinse the coated surfaces contains an acidulated rinse material.

3. A method according to claim 1 and further comprising sealing said coatings by applying an acidulated final rinse thereto following said rinsing with water.

4. A method according to claim 1 in which said alkaline cleaner is selected from the class consisting of orthophosphate, caustic, and silicate based cleaners.

5. A method according to claim 1 and further comprising applying an activating material to said surfaces prior to contacting them with phosphating solution.

6. A method according to claim 5 in which said acti vat'ing material is applied simultaneously with said alkaline cleaner.

reaches about 5 g/l. 

2. A method according to claim 1 wherein the water used to rinse the coated surfaces contains an acidulated rinse material.
 3. A method according to claim 1 and further comprising sealing said coatings by applying an acidulated final rinse thereto following said rinsing with water.
 4. A method according to claim 1 in which said alkaline cleaner is selected from the class consisting of orthophosphate, caustic, and silicate based cleaners.
 5. A method according to claim 1 and further comprising applying an activating material to said surfaces prior to contacting them with phosphating solution.
 6. A method according to claim 5 in which said activating material is applied simultaneously with said alkaline cleaner.
 7. A method according to claim 1 in which a plurality of alkaline cleaners are sequentially applied to said surfaces.
 8. A method according to claim 1 in which said phosphating solution is a chlorate-nitrite accelerated solution.
 9. A method according to claim 1 in which said soil content maintenance is effected by discarding the alkaline cleaner bath and replacing it when the soil content reaches about 5 g/l. 